Doesn't have to be 100% clean, but history has tremendous value to me. Often the most important question for me when fixing a problem is "was this intentional" (aka did that guy (who left 10 years before I arrived) know what he was doing?).

As far as I remember, I've had to check history for about 2/3 of bugs I've fixed.

In a perfect world each system would be documented and tested well enough that correctness could be derived from first principles, but we do not live in a perfect world :(